1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as power transformers, and more specifically to electrical windings for such apparatus which have a high series capacitance.
2. Description of the Prior Art
Electrical inductive apparatus, such as single and polyphase electrical power transformers of the coreform type, commonly utilize a high voltage phase winding which includes a plurality of electrically connected pancake- or disc-type coils arranged in an axially aligned stack about a winding leg of a magnetic core. A surge potential, such as caused by lightning or switching, applied to the line terminal of a winding of this type, distributes itself across the turns of the pancake coils, across the winding, and from the winding to ground according to the capacitive structure of the winding, with the conductors and ground being the "electrodes" of the capacitors, and the winding insulation, and other insulating members, providing the dielectric. It is characteristic of the pancake coil type winding for a surge potential to concentrate at the line end of the winding, and rapidly attenuate as it enters the winding. It is desirable to distribute such surges as uniformly as possible across the turns of the pancake coils, and across the pancake coils of the winding, in order to prevent the stress from building up to undesirably high values, which may cause the stressed insulation to fail. Further, it is desirable to uniformly distribute surge potentials in order to reduce the magnitude of transient voltage oscillations produced when the voltage distribution changes from capacitive to inductive. The more nearly the capacitive voltage distribution conforms to the inductive distribution, the lower the magnitude of transient voltage oscillations produced as the distribution changes from capacitive to inductive.
An indication of how uniformly a surge potential will be distributed across a winding may be obtained from the distribution constant alpha of the winding. The distribution constant alpha is equal to the square root of the ratio of the capacitance C.sub.g of the winding to ground to the through or series capacitance C.sub.s of the winding. ##EQU1## The smaller the distribution constant alpha, the more uniformly a surge voltage will be distributed across the winding. Since the distribution constant alpha may be reduced by increasing the series capacitance of the winding, it is common in the prior art to form the pancake coils by simultaneously winding two or more conductors to form a plurality of coil sections, the turns of which are radially interleaved. Then, by connecting the sections of the pancake coils to mechanically locate turns from an electrically distant part of the coil or winding, between electrically connected turns, called interleaving, the voltage between physically adjacent coils is increased and adjacent turns are effectively connected in parallel, which increases the through or series capacitance of each pancake coil, and of the electrical winding.
Many different interleaving arrangements are used in the prior art. Certain of the arrangements are necessary in order to achieve different degrees of interleaving, and thus different values of series capacitance as required by specific applications, or in different sections of a single winding. Other arrangements are necessary in order to achieve interleaved type windings while utilizing two or more electrical conductors which are connected in parallel with one another, in order to increase the current carrying capacity of the winding.
The desired current carrying capacity and loss rating of the electrical inductive apparatus determine the conductor cross-sectional area for the winding turns, and the number of parallel-connected strands to achieve this cross-sectional area. Transformer losses are becoming of greater importance to electrical utilities, and thus transformers are being designed to achieve lower losses. The cross-sectional area of the conductive portion of a conductor turn is increased in order to reduce I.sup.2 R losses. The required cross-sectional area is provided by a plurality of strands, instead of being provided in one large conductor, in order to reduce losses due to eddy currents. Thus, the windings for a given power rating are increasing in physical size. Increasing the physical size of a winding greatly increases the manufacturing cost of such apparatus, beyond the extra cost of the added conductor material, because it increases the size of the associated magnetic core, which, in turn, requires a larger tank, and the larger tank requires more liquid dielectric. When the winding whose physical size is increased is an inner winding of concentrically adjacent windings, the manufacturing cost escalation is particularly steep, with even very small increases in the diameter of an inner winding resulting in large increases in manufacturing cost, as the outer windings then must have larger inside diameters, greatly increasing their outside dimensions, with the associated increases in the size of the magnetic core and tank.
Thus, when the transformer is designed for the required impedance, current carrying capacity, and losses, using the list of discrete wire sizes available, it may require that each coil section have an average predetermined number of conductor turns, plus a half conductor turn, in order to provide the required volts per turn over a predetermined number of pancake coils having a predetermined axial stack dimension and predetermined radial build dimension.
Certain high BIL-rated inner windings must have a high series capacitance in order to prevent surge concentrations, as hereinbefore set forth. The current carrying capacity and loss rating may require that this high series capacitance inner winding have four strands or four parallel-connected conductive paths. If the design then dictates a half turn, it would be extremely difficult to economically construct such as winding using prior art techniques, and obtain a mechanically strong structure having a uniform, minimum build dimension.